1. Field of the Invention
The present invention relates to ophthalmic lenses, and more particularly to contact lenses which are rotationally stabilized. Specifically the present invention relates to designs and methods which optimize non-circularity and thickness differential to improve comfort while ensuring satisfactory rotational stabilization.
2. Discussion of the Related Art
Contact lenses are considered medical devices and may be worn to correct vision and/or for cosmetic or other therapeutic reasons. Contact lenses have been utilized commercially to improve vision since the 1950s. Early contact lenses were made or fabricated from hard materials, and were relatively expensive and fragile. Although these contact lenses are still utilized, they are not suitable for all patients due to their poor initial comfort. Later developments in the field gave rise to soft contact lenses, based upon hydrogels, which are extremely popular and widely utilized today. The introduction of soft contact lenses has significantly improved the comfort experienced by the wearer. One type of contact lens is a spherical contact lens, which for the most part provides a uniform power, and since these lenses are substantially spherical they are rotationally symmetric so rotational placement or positioning on the eye is not a major concern. For those patients who require an astigmatic correction, one can provide a cylinder optical power correction in addition to the spherical power correction to achieve vision correction. These lenses, sometimes referred to as toric lenses, require the optical design to be rotationally stabilized in the correct orientation while worn on eye. In the astigmatic patient, relative rotational orientation of the lens is important to correct one's vision.
Astigmatism is caused by a non-rotationally symmetric curvature of the cornea and/or the crystalline lens. A normal cornea is substantially rotationally symmetric, whereas in an individual with astigmatism this is not the case. In other words, the cornea of the eye is actually more curved or steeper in one direction than another, thereby causing an image to be stretched out into a line of focus rather than focused to a single point. Toric rather than spherical/single vision lenses can be used to address this. A toric lens is an optical element having two different target powers in two orientations that are perpendicular to one another. Essentially, a toric lens has one power, spherical for correcting myopia or hyperopia, and one power, cylinder, at a particular axis, for correcting astigmatism, both powers built into a single lens. These powers are created with curvatures oriented at different angles which are preferably maintained relative to the eye. The proper rotational orientation of the toric lens is thus essential to properly correct for astigmatism. However, with use, toric contact lenses may tend to rotate on the eye thereby temporarily providing sub-optimal vision correction. Accordingly, currently utilized toric contact lenses also include a mechanism to keep the contact lens relatively stable and in the proper orientation on the eye when the wearer blinks or looks around in order to maintain the correct vision correction. This mechanism also serves to return the lens to the stable and proper orientation on eye after insertion or when the lens moves out of the proper position and orientation. To ensure the proper orientation of the lens, various methods of lens stabilization have been utilized in the art such as ballast or preferential thick and thin zones. While there are various ways to achieve stabilization, all ways ultimately will be affected to varying degrees, by the interaction of the posterior surface of the contact lens with that of the anterior surface of the eye, and the eyelids as well, particularly in the peripheral regions, which may also negatively impact vision and or subjective wearer comfort.
The challenge with currently designed or utilized stabilization zones is a tradeoff between contact lens stability and comfort, as well as the physical limitations associated with increased thickness. Changes to the design to improve rotational speed, such as increasing the surface slope of the stabilization zone, also increases contact lens thickness and may adversely impact comfort. Additionally, the contact lens design has to accomplish two things; namely, to rotate to the proper orientation on insertion, and to maintain that orientation through the wear period. Conventional designs require tradeoffs in performance between these multiple considerations.
In U.S. Pat. No. 6,406,145, rotationally stabilized contact lenses with minimal thickness changes are disclosed. In U.S. Pat. No. 6,491,392 the design of the stabilization features incorporates the use of spline or polynomial functions in order to improve comfort, while U.S. Pat. Nos. 6,939,005 and 7,159,979 focus on the rate of change of the thickness differential in order to reduce the time to stabilize the lens. U.S. Pat. Nos. 7,201,480 and 7,682,019 employ the use of thin zones for stabilization purposes.
In more recent attempts to address the needs of the astigmatic patient, for example, U.S. Pat. No. 8,827,448 proposes the use of astigmatism-free customized lenses for refractive correction with a first cylindrical power on the anterior surface and a second cylindrical power on the posterior surface of the contact lens. While it is suggested that improved visual acuity is achieved with such a design, these items are limited to the optical zone of the lens and how that interacts with an asymmetrically shaped cornea. Design changes in other regions, most notably the peripheral region of the lens, can still have an impact and would not adversely impact those limited to the optical zone seeking to improve visual acuity and thus coexist and further improve lens performance.
Early use of non-round lenses for stabilization purposes is disclosed in U.S. Pat. No. 5,760,870, primarily as a way to avoid thickening the lens for stabilization purposes. The inventors of the '870 patent indicated that lens thickening resulted in patient discomfort, undesirable power variations, and reduced oxygen permeability in the thickened regions, advocating the use of a non-round lens instead and the resulting difference in the aspect ratio of the lens to achieve stabilization rather than thickness differential. More recently, in U.S. Pat. No. 8,668,331 the use of a non-round lens to maximize the lens-lid interaction for the purposes of centration, rotation, and stabilization is disclosed and can be instructive. In US Published Patent Application No. US20140063444 assigned to the same assignee as the present invention, the use of round and non-round shapes along with stabilization zones is disclosed; however optimizing the shape and thickness in combination is not discussed.
In summary, previous innovators in this space have attempted to address this issue of stabilization by selective lens thickening, lens thinning, prism ballast, and other methods all of which can be generally characterized as thickness differential designs, while others looked for a solution by the use of non-round lenses, which may be generally characterized as the circularity/non-circularity designs, which in some cases attempted to mask the astigmatism altogether, however, none to date have looked to combine these methods in an optimized fashion in a single lens design to achieve rotational stabilization with improved comfort. Depending on the approach one uses, and because it is a balance between rotational stability and comfort, it is believed there is an opportunity that exists by combining the best from these approaches.
Accordingly, there exists a need for contact lenses with rotational stability achieved by simultaneously optimizing non-circularity and thickness differential as a system in order to achieve improved on eye performance while maintaining a high degree of comfort and vision correction.